The principal thrust of this research program remains unchanged. Its theoretical foundation is an ontogenetic approach to the study of brain-behavior relations. Its specific objective is to investigate the structural and functional role of late-forming, short-axoned interneurons in selected brain regions. Low-level x-irradiation is the major experimental tool used. With it we can prevent the formation of certain classes of interneurons in specific brain regions. The effects of these cellular dissections are examined on the subsequent organization of the brain structures concerned and on their behavioral manifestations during the juvenile, young-adult and adult periods in rats. Normal and x-irradiated brains, beginning at the early phases of embryonic development through adulthood, are prepared for light microscopy (including 3H-thymidine autoradiography and the Golgi technique) and electron microscopy. With the aid of an operator-guided computer system quantitative studies will be made of the prenatal development of the cerebellum, hippocampus, olfactory bulb and cerebral cortex. This will be followed by a correlated examination of the pre-and postnatal development of the principal components of the visual system, limbic system and olfactory system. The effects of cerebellar reorganization will be examined on the development of motor skills (traversing paths graded in difficulty), on motor strategies (hyper-activity and response habituation) and on spatial and motor learning. The effects of hippocampal granule cell agenesis will be studied on the acquisition and reversal of maze habits varied in terms of cue modality, cue difficulty and the need for orientation toward the cue.